Attention-deficit hyperactivity disorder (ADHD) is defined by symptoms of inattention, impulsivity and hyperactivity, but the existence of all three concurrently is not required for a diagnosis. ADHD children also differ in associated features, such as the presence of aggressive behavior or cognitive deficits, and have disparate long-term outcomes. Due in part to this heterogeneity, diagnostic validity for ADHD has not been established, and as a result, it has been impossible to isolate the etiologic and prognostic determinants of this extremely prevalent disorder. It is proposed that ADHD is not a unitary or a valid syndrome, but rather, that two distinct groups of ADHD children can be identified based upon the presence o absence of aggressive behavior. It is hypothesized that non- aggressive ADHD children will comprise an Inattentive/Cognitively Impaired group characterized by inattention, impulsivity, cognitive deficits, and evidence of increased noradrenergic metabolism; and aggressive ADHD children will comprise an Impulsive/Aggressive group characterized by impulsivity, in the absence of inattention and cognitive problems, and evidence of reduced central serotonergic function and noradrenergic metabolism. The goal is to establish the concurrent validity of the diagnostic distinctions between these two groups of ADHD children. Three groups of 30 boys will participate in this research: aggressive ADHD boys, non-aggressive ADHD, and normal controls. The three groups will be compared on neuropsychological measures of attention, impulse control, and learning. The two patient groups will be compared on neurobiological measures of noradrenergic and serotonergic functioning.